 In the seventh book of the series, the concept of an impending millennial apocalypse is introduced.Although the theme of the "soulmate principle" continues to be present, the later books' plots focus on the search for four "Wild Powers" who, according to an ancient prophecy, will either save the world or aid in its destruction.Books from the series appeared on the "New York Times" Bestseller List for children's books in 2008 and 2009.